The proliferation of flat panel displays into increasing numbers of applications places higher demands on the performance of all display components. Flat panel displays based on liquid crystal switching modes and electroluminescent technology both depend on the use of polarizing plates to improve the image quality. With the increasing penetration of flat panel displays into mobile and outdoor application comes the demand for improved environmental stability of all components.
One component of displays that have long been known to have severe restrictions on environmental durability, in particular with respect to exposure to elevated temperature and humidity, are polarizing plates. Polarizing plates are multi-layer structures comprising an oriented polarizing film. Polarizing films require a protective covering to maintain stability of the polarizing film. Conventionally, protective polymer films 1 have been laminated to both faces of a polarizing film 2 as shown in FIG. 1. The most commercially successful class of polarizing films comprises oriented polyvinyl alcohol (PVA) that has been dyed to provide polarizing activity in the visible light spectrum. The protective films are required to be optically transparent, dimensionally stable, mechanically tough, and chemically compatible with polarizing films. The most commercially successful of these protective films has been cellulose triacetate (commonly known in the polarizer industry as triacetyl cellulose or TAC), although other polymers may be used.
For the purposes of this invention the following terms are defined:
A “polarizing film” is defined herein as a self-bearing oriented polymer film with associated addenda that performs the function of polarizing light. For example, oriented PVA film that has been dyed by complexation with dichroic organic compounds or iodine and cross-linked with borate, including ancillary addenda, is an effective polarizer film. Henceforth herein it is understood that the term “dye” includes dichroic organic compounds and iodine, except as otherwise specified.
A “protective film” is defined herein as a self-bearing polymer film with associated addenda, such as plasticizers, UV absorbers, stabilizers, etc., that is laminated, adhered to, coated, or otherwise applied onto the polarizing film to provide protection for the polarizing film. Protective functions can include providing stability in terms of mechanical, optical, chemical, or other properties and resistance to environmental degradation. The durability of polarizing plates is conventionally measured based on retention of their polarization efficiency and light-transmission performance when exposed to elevated temperature and/or humidity environments for extended time periods.
A “protective film” can comprise part of a composite film including, for example, a carrier layer. A “polarizing plate” is defined herein as the multilayer polymer film structure consisting of at least one polarizing film and at least one protective film. The invention is particularly advantageous when the protective film is the first self-bearing film contiguous to the polarizing film in a polarizing plate.
Previous approaches have been described which attempt to improve the environmental durability of polarizing plates. For the most part these efforts have focused on controlling the moisture content and moisture permeability of the protective films in the polarizing plate (See, for example, US 20020192397A1, US 20020162483A1, and US 20030037703A1). These approaches have attempted to control the moisture permeation properties through the use of high levels of organic compounds known to plasticize the protective film. For the purposes of this invention a plasticizing compound or “plasticizer” is defined as a compound that is chemically compatible with the polymer and reduces brittleness or imparts improved flexibility and elongation to the film.
In support of the present invention, intensive study was undertaken to determine the key factors compromising the durability of polarizer plates under environmental stress, i.e., elevated temperature and/or humidity. This study has shown that acceptable moisture permeation of the protective film, as defined for example in US patent application 20020192397A1, allows polarizer plates to rapidly achieve moisture uptake equilibrium. The key factor contributing to long-term failure of polarization efficiency and light transmission performance of polarizer plates was determined to depend on the release of active chemical species, particularly those that are mobile. In particular, these species were found to be related to the breakdown products of plasticizer and other protective film components. These findings are supported by prior work as found, for example, in: Shinigawa et al. “Investigation of the Archival Stability of Cellulose Triacetate Film: The Effect of Additives to CTA Support,” Polym. Conserv., 105 (1992), 138-150; and Ram et al. “The Effects and Prevention of the ‘Vinegar Syndrome’,” J. Imaging Sci. and Tech., 38 (1994), 749-761, all hereby incorporated by reference.
Prior attempts to improve the durability of polarizer plates have been described. US patent application 20020192397A1 discloses the use of elevated levels of phosphate ester plasticizers to control the moisture permeability of the protective film. However, as noted by Ram et al. and Shinigawa et al., phosphate ester plasticizers release strongly acidic compounds upon exposure to elevated humidity. In addition, chemically active hydroxy arenes, such as phenols, are released.
The use of a small-molecule basic compound (tributyl amine) is also disclosed in US 20020192397A1 to reduce the odor of acetic acid in TAC protective films. Small-molecule basic compounds are not preferred, however, as they readily migrate into the polarizer film and are reactive toward the polarizer dye components.
The use of ester compounds that can release chemically active hydroxy arenes, such as phenol, is also not preferred. The current study has shown that undesirable reaction with iodine in the polarizer film to produce iodophenols occurs in polarizer plates that have been subjected to elevated temperature and humidity when hydroxy arene ester plasticizers are employed. The reactivity of hydroxy arenes toward halogens is well known, as described by Morrison, R. T. and Boyd, R. N., Organic Chemistry, 3rd Ed., Allyn and Bacon, Boston, 1976, pp. 801-802, hereby incorporated by reference.
US 20020162483A1 and U.S. Pat. No. 5,753,140 disclose the use of elevated levels of aromatic ester plasticizers to control moisture permeability and moisture content in protective films for polarizer plates. Such compounds release strongly acidic compounds and/or chemically active hydroxy arenes upon exposure to elevated humidity. US patent application 20030037703A1 discloses the use of elevated levels of monocarboxylic acid esters of polyhydric alcohols as plasticizers to control moisture permeability of protective films for polarizer plates. Due to the use of esters of monocarboxylic acids, again elevated humidity conditions lead to the production of mobile acidic compounds.
In the cases described above (US 20020192397A1, US 20020162483A1, and US 20030037703A1), the acidic species and hydroxy arenes produced have the potential to readily migrate through the polarizer plate structure. Strongly acidic species can participate in the degradation of the protective films, the adhesion interface to the polarizing film, and most critically, the organic dyes in the polarizing film. Interaction of the dyes in the PVA polarizing film with such reactive species can produces significant hue shift contributing to degradation of the polarizer plate performance.
In addition, in the cases described above (US 20020192397A1, US 20020162483A1, US 20030037703A1) high levels of these compounds (>15% by weight) are incorporated into the polymer lamination films in order to provide significant control of moisture permeability and moisture content. These high levels are undesirable as they lead to unacceptable loss of toughness by the protective film through reductions in polymer modulus (Handbook of Plasticizers, G. Wypych, ed., ChemTec Publishing, NY, 2004, Chap. 10).